This paper describes an electrooptic probing technique using a CW semiconductor-laser beam associated with a fast photodetector. Besides its simplicity, this technique presents some advantages over the sampling one thanks to the presence of a Fabry-Perot effect, namely an enhancement of the electrooptic interaction and a simple solution to the calibration problem. The good validity of the calibration method allows the application of this technique to S-parameter measurements. The S-parameter determination, in modulus and in phase, of an industrial MMIC by the electrooptic method is reported and compared with direct network analyzer measurements. >